Polythioether polymers are widely used in aviation and aerospace sealants primarily due to their excellent fuel-resistance. In addition to resistance to aviation fuels, polymers useful in aviation and aerospace sealants desirably exhibit the properties of low temperature flexibility, room temperature liquidity, and high temperature resistance. It is also desirable that the process used to synthesize the polythioether polymers be low cost, and free of malodorous and acidic byproducts. Developments in polythioether polymer chemistry have led to polymers exhibiting properties suitable for aviation and aerospace applications. For example, polythioether polymers formed by the free radical catalyzed addition reaction of vinyl ethers and polythiols as disclosed in U.S. Pat. No. 6,172,179, U.S. Pat. No. 5,959,071, and U.S. Pat. No. 5,912,319 are liquid at room temperature, exhibit excellent low-temperature flexibility and fuel resistance, and the synthesis does not generate undesirable cyclic or acidic byproducts.
It is further desirable that polythioether polymers used in aviation and aerospace sealants remain liquid at low temperatures potentially encountered, for example, during transportation and storage. Specifically, it is desirable that the polythioether polymers remain liquid at a temperature of 20° C. (68° F.), and more preferably at a temperature of 4° C. (39° F.), for an extended period of time.
In polythioether polymer systems it is known that the introduction of non-linearity into the polymer backbone, such as by incorporating pendent groups, reduces the glass transition temperature of the polymer and enhances the ability of the polymers to remain liquid at low temperatures. U.S. Pat. No. 4,366,307 discloses the incorporation of pendent alkyl side chains to provide liquid polythioether polymers with a glass transition temperature less than −50° C. U.S. Pat. No. 5,959,071 discloses incorporating pendent methyl groups into polythioether polymers to produce fuel resistant polymers that are liquid at low temperatures and that exhibit a glass transition temperature less than −50° C.
Polythioether polymers formed by the two-step addition reaction of polythiol, polyepoxide, and polyvinyl ether are disclosed in U.S. Pat. No. 6,486,297. In a first step, a polythiol is reacted with either a polyepoxide or a polyvinyl ether to form a prepolymer. In a second step, the prepolymer and un-reacted polythiol is reacted with the component not participating in the first reaction step. The polyepoxide reaction introduces pendent hydroxyl groups along the backbone of the polythioether polymer and thereby increases the non-linearity in the polymer backbone. Polythioether polymers produced using polyepoxides as disclosed in U.S. Pat. No. 6,486,297 exhibit a glass transition temperature less than −40° C. However, because the polyepoxide reaction favors polymer chain extension during the reaction, the resulting polythioether polymers are characterized by a high molecular weight and exhibit commensurate high viscosities on the order of 400 poise at room temperature. For use of polythioether polymers in curable sealant compositions, it is desirable that the polymer viscosity be on the order of 100 poise or less at room temperature.
To overcome the disadvantages inherent in polythioether polymers synthesized using polyepoxides while maintaining the properties advantageous for aviation and aerospace sealant applications, a three-step method using monoepoxides for the synthesis of polythioether polymers and sealants made therefrom, are herein disclosed.
Use of thiol addition chemistry in a three-step reaction process enables control of the polymer structure leading to polythioether polymers that exhibit low-temperature liquidity, as well as other properties desirable for aviation and aerospace sealant applications.
In a first reaction step, a polythiol can be reacted with a monoepoxide having an epoxy group and a second group, other than an epoxy group, that is reactive with a thiol group, such that the reaction takes place preferentially at the second group, to form a first prepolymer. In the first step, a thiol group adds across the double bonds of the second, non-epoxy, group to form the first prepolymer. The first prepolymer can be the 1:1 addition product of a polythiol and a monoepoxide, and comprises an epoxy group and a thiol group. Following the first reaction step, the reaction mixture comprises the first prepolymer and un-reacted polythiols.
The second reaction step comprises the ring opening of the epoxy groups by un-reacted thiol groups, typically in the presence of a catalyst, to form a second prepolymer. In the second reaction step, thiol groups on both the first prepolymer and un-reacted polythiols participate in the ring opening of the epoxy groups to form the second prepolymer. After the completion of the second reaction step, the reaction mixture comprises the second prepolymer and un-reacted starting polythiols. The second prepolymer is a polythiol having a higher molecular weight than the starting polythiols.
The third reaction step comprises the free radical-catalyzed addition of the thiol groups of both the second prepolymer and remaining un-reacted starting polythiols across the double bonds of a polyunsaturated compound such as a divinyl compound.
The three-step synthesis enables control of the molecular weight, polymer structure, and the equivalent weight, to produce polythioether polymers with consistent chemical and physical properties, and existing as a liquid at a temperature of 20° C. or less and that are useful for aviation and aerospace sealant applications. The controlled introduction of polar hydroxyl groups into the backbone of the polythioether polymer, by increasing the overall polarity of the polymer without undesirable chain extension, enhances the compatibility of the polythioether polymer with additives used in the formulation of useful sealant compositions, and also enhances the adhesion properties of the polythioether polymer to surfaces.